Re: [PATCH 4/4] xen/uart: enable parsing ACPI SPCR on x86

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 26/03/2026 12:48 pm, Jan Beulich wrote:
> On 26.03.2026 13:11, Andrew Cooper wrote:
>> On 25/03/2026 2:58 pm, Roger Pau Monne wrote:
>>> Introduce extra logic to allow parsing ACPI tables extra early, and use it
>>> to parse the ACPI SPCR table and obtain the serial configuration.
>>>
>>> This is gated to the "acpi" device type being set in "com1" on the Xen
>>> command line.  Note that there can only be one serial device described in
>>> the SPCR, so limit it's usage to com1 exclusively for the time being.
>>>
>>> I can't test the interrupt information parsing on my system, as the
>>> interrupt is set to GSI with a value of 0xff, which is outside of the range
>>> of GSIs available on the system.  I've also assumed that the interrupt
>>> being 0xff is used to signal not interrupt setup (just like the Interrupt
>>> Pin register on PCI headers).
>>>
>>> Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
>>> ---
>>> WIP/RFC, not sure whether there's interest in attempting to pursue this
>>> further on x86.  So far the device I have is also exposed on the PCI bus
>>> aside from SPCR, so using com1=device=amt also works to detect it.
>>>
>>> Posting it kind of early to know whether I should try to polish it for
>>> submission or we are happy with not having this on x86.
>> I think we should be using SPCR/DBG2 when available.  Getting serial
>> configuration right is always tricky, and we might as well use the help
>> that Microsoft have forced the OEM/firmware world to provide.
>>
>> But, I think it should be automatic when the user asked for any kind of
>> serial.  e.g. console=com1 with no com1 configuration.  The point of
>> these tables is to provide an enumeration mechanism where none
>> previously existed.
> Hmm. In the PC world COM<n> have well-known configurations unless anything
> else is provided. With multiple serial ports in a system, which one SPCR
> describes also would be (largely) unknown.

Xen's COM1/2 already do do far more than the PC world.  But ok then, we
invent a new "serial".

My point is, there should be a way to say "please use serial as
described by the system", and it shouldn't even require knowing that the
description is in APCI.

~Andrew